What would happen would be a speed tie, where there is an equal chance of either Pokemon going first.

The Speed stat determines how quickly a Pokémon can act in battle.
Pokémon with higher Speed will make a move before ones with lower
Speed under normal conditions; in the case that two Pokémon have the
same Speed, one of them will randomly go first. Deoxys in its Speed
Forme currently has the highest Speed among all Pokémon with 180.
Ninjask currently has the highest Speed of all non-legendary Pokémon
with 160. The average Speed Stat is 66 for all Pokémon, and the
average for all fully evolved Pokémon is 78.